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Chemical mechanisms form the core of atmospheric models to describe degradation pathways of pollutants and ultimately inform air quality and climate policymakers and other stakeholders. The accuracy of chemical mechanisms relies on the quality of their input data, which originate from experimental (laboratory, field, chamber) and theoretical (quantum chemistry, theoretical kinetics, machine learning) studies. The development of robust mechanisms requires rigorous and transparent procedures for data collection, mechanism construction and evaluation and the creation of reduced or operationally defined mechanisms. Developments in analytical techniques have led to a large number of identified chemical species in the atmospheric multiphase system that have proved invaluable for our understanding of atmospheric chemistry. At the same time, advances in software and machine learning tools have enabled automated mechanism generation. We discuss strategies for mechanism development, applying empirical or mechanistic approaches. We show the general workflows, how either approach can lead to robust mechanisms and that the two approaches complement each other, resulting in reliable predictions. Current challenges are discussed related to global change, including shifts in emission scenarios that result in new chemical regimes (e.g., low-NO scenarios, wildfires, mega- and gigacities) and that require the development of new or expanded gas- and aqueous-phase mechanisms. In addition, new mechanisms should be developed to also target oxidation capacity and aerosol chemistry impacting climate, human and ecosystem health.

The budget of reactive nitrogen species, which play a central role in atmospheric chemistry (e.g. in photochemical O3 production), is poorly understood in forested regions. In this study, through observations of NO, NO2, NOy, and O3 in the Rambouillet Forest near Paris, France, we have examined nighttime processes controlling NOx in an anthropogenically impacted forest environment. The O3 mixing ratios displayed a strong diel profile at the site that was driven by a variable but generally rapid deposition to soil and foliar surfaces. The O3 diel profile was strongly influenced by relative humidity and temperature inversion. Only when the O3 mixing ratio was sufficiently low (and thus the NO lifetime sufficiently long) were sustained NO peaks observed above the instrumental detection limit, enabling the derivation of average NO emission rates of ∼1.4 ppbv h−1 from the soil. Observations of the lack of increase in NO2 at night, despite a significant production rate from the reaction of NO with O3, enabled an effective lifetime of NO2 of ∼0.5–3 h to be derived. As the loss of NO2 was not compensated for by the formation of gas- or particle-phase reactive nitrogen species, it was presumably either driven by deposition to soil and foliar surfaces or any products formed were themselves short-lived with respect to deposition. By comparison, the daytime lifetime of NO2 with respect to loss by reaction with OH is about 1 d. Our results indicate that the nighttime deposition of NO2 is a major sink of boundary layer NOx in this temperate forest environment.

​​​​​​​Volatile organic compounds (VOCs) play a key role in tropospheric chemistry, giving rise to secondary products such as highly oxygenated organic molecules (HOMs) and secondary organic aerosols (SOAs). HOMs, a group of low-volatility gas-phase products, are formed through the autoxidation process of peroxy radicals (RO2) originating from the oxidation of VOCs. The measurement of HOMs is made by a NO3- ToFCIMS instrument, which also detects other species like small highly oxygenated VOCs (e.g., dicarboxylic acids) and sulfuric acid (H2SO4). The instrument response to HOMs is typically estimated using H2SO4, as HOMs are neither commercially available nor easily synthesized in the laboratory. The resulting calibration factor is then applied to quantify all species detected using this technique. In this study, we explore the sensitivity of the instrument to commercially available small organic compounds, primarily dicarboxylic acids, given the limitations associated with producing known amounts of HOMs for calibration. We compare these single-compound calibration factors to the one obtained for H2SO4 under identical operational conditions. The study found that the sensitivity of the NO3- ToFCIMS varies depending on the specific type of organic compound, illustrating how a single calibration factor derived from sulfuric acid is clearly inadequate for quantifying all detected species using this technique. The results highlighted substantial variability in the calibration factors for the tested organic compounds, with 4-nitrocatechol exhibiting the highest sensitivity and pyruvic acid the lowest. The obtained sulfuric acid calibration factor agreed well with the previous values from the literature. In summary, this research emphasized the need to develop reliable and precise calibration methods for progressively oxygenated reaction products measured with a NO3- chemical-ionization mass spectrometer (CIMS), for example, HOMs.

We present direct measurements of biogenic volatile organic compound (BVOC)-induced nitrate radical (NO3) reactivity (kVOC) through the diel cycle in the suburban, temperate forest of Rambouillet near Paris (France). The data were obtained in a 6-week summer period in 2022 as part of the Atmospheric ChemistRy Of the Suburban foreSt (ACROSS) campaign. kVOC was measured in a small (700 m2) clearing mainly at a height of 5.5 m above ground level but also at 40 m (for 5 d and nights). At nighttime, mean values (and 25th–75th percentile ranges) of knightVOC(5.5m) = (0.24-0.06+0.32) s−1 and knightVOC(40m) = (0.016-0.007+0.018) s−1 indicate a significant vertical gradient and low NO3 reactivity above the canopy, whereas knightVOC(5.5 m) showed peak values of up to 2 s−1 close to the ground. The strong vertical gradient in NO3 reactivity could be confirmed by measurements between 0 and 24 m on one particular night characterized by a strong temperature inversion and is a result of the decoupling of air masses aloft from the ground- and canopy-level sources of BVOCs (and nitric oxide, NO). No strong vertical gradient was observed in the mean daytime NO3 reactivity, with kdayVOC(5.5m) = (0.12 ± 0.04) s−1 for the entire campaign and kdayVOC(40m) = (0.07 ± 0.02) s−1 during the 5 d period. Within the clearing, the fractional contribution of VOCs to the total NO3 loss rate coefficient (ktot, determined by photolysis, reaction with NO and VOCs) was 80 %–90 % during the night and ∼ 50 % during the day. In terms of chemical losses of α-pinene below canopy height in the clearing, we find that at nighttime hydroxyl radicals (OH) and ozone (O3) dominate, with NO3 contributing “only” 17 %, which decreases further to 8.5 % during the day. Based on measured OH, measured O3, and calculated NO3 concentrations, the chemical lifetime of BVOCs at noon is about 1 h and is likely to be longer than timescales of transport out of the canopy (typically of the order of minutes), thus significantly reducing the importance of daytime in-canopy processing. Clearly, in forested regions where sufficient nitric oxide and nitrogen dioxide (NOx) is available, the role of NO3 and OH as initiators of BVOC oxidation is not strictly limited to nighttime and daytime, respectively, as often implied in e.g. atmospheric chemistry textbooks.

Biogenic volatile organic compounds (BVOCs) are intensely emitted by forests and crops into the atmosphere. They can rapidly react with the nitrate radical (NO3) during the nighttime to form a number of functionalized products. Among them, organic nitrates (ONs) have been shown to behave as reservoirs of reactive nitrogen and consequently influence the ozone budget and secondary organic aerosols (SOAs), which are known to have a direct and indirect effect on the radiative balance and thus on climate. Nevertheless, BVOC + NO3 reactions remain poorly understood. Thus, the primary purpose of this study is to furnish new kinetic and mechanistic data for one monoterpene (C10H16), terpinolene, and one sesquiterpene (C15H24), β-caryophyllene, using simulation chamber experiments. These two compounds have been chosen in order to complete the few experimental data existing in the literature. Rate constants have been measured using both relative and absolute methods. They have been measured to be (6.0 ± 3.8) ×10-11 and (1.8 ± 1.4) ×10-11 cm3 molec.−1 s−1 for terpinolene and β-caryophyllene respectively. Mechanistic studies have also been conducted in order to identify and quantify the main reaction products. Total organic nitrates and SOA yields have been determined. Both terpenes appear to be major ON precursors in both gas and particle phases with formation yields of 69 % for terpinolene and 79 % for β-caryophyllene respectively. They are also major SOA precursors, with maximum SOA yields of around 60 % for terpinolene and 90 % for β-caryophyllene. In order to support these observations, chemical analyses of the gas-phase products were performed at the molecular scale using a proton transfer reaction–time-of-flight–mass spectrometer (PTR-ToF-MS) and FTIR. Detected products allowed proposing chemical mechanisms and providing explanations through peroxy and alkoxy reaction pathways.

Residential biomass burning for heating purposes is an important source of air pollutants during winter. Here we test the hypothesis that significant secondary organic aerosol production can take place even during winter nights through oxidation of the emitted organic vapors by the nitrate (NO3) radical produced during the reaction of ozone and nitrogen oxides. We use a mobile dual smog chamber system which allows the study of chemical aging of ambient air against a control reference. Ambient urban air sampled during a wintertime campaign during nighttime periods with high concentrations of biomass burning emissions was used as the starting point for the aging experiments. Biomass burning organic aerosol (OA) was, on average, 70 % of the total OA at the beginning of our experiments. Ozone was added in the perturbed chamber to simulate mixing with background air (and subsequent NO3 radical production and aging), while the second chamber was used as a reference. Following the injection of ozone, rapid OA formation was observed in all experiments, leading to increases in the OA concentration by 20 %–70 %. The oxygen-to-carbon ratio of the OA increased on average by 50 %, and the mass spectra of the produced OA was quite similar to the oxidized OA mass spectra reported during winter in urban areas. Furthermore, good correlation was found for the OA mass spectra between the ambient-derived emissions in this study and the nocturnal aged laboratory-derived biomass burning emissions from previous work. Concentrations of NO3 radicals as high as 25 ppt (parts per trillion) were measured in the perturbed chamber, with an accompanying production of 0.1–3.2 µg m−3 of organic nitrate in the aerosol phase. Organic nitrate represented approximately 10 % of the mass of the secondary OA formed. These results strongly indicate that the OA in biomass burning plumes can chemically evolve rapidly even during wintertime periods with low photochemical activity.

Biogenic volatile organic compounds (BVOCs) are intensely emitted by forests and crops into the atmosphere. During the night, they react very rapidly with the nitrate radical (NO3), leading to the formation of a variety of functionalized products including organic nitrates and to large amounts of secondary organic aerosols (SOAs). Organic nitrates (ONs) have been shown not only to play a key role in the transport of reactive nitrogen and consequently in the ozone budget but also to be important components of the total organic-aerosol mass, while SOAs are known to play a direct and indirect role in the climate. However, the reactivity of BVOCs with NO3 remains poorly studied. The aim of this work is to provide new kinetic and mechanistic data for two monoterpenes (C10H16), α- and γ-terpinene, through experiments in simulation chambers. These two compounds, which have very similar chemical structures, have been chosen in order not only to overcome the lack of experimental data but also to highlight the influence of the chemical structure on the reactivity. Rate constants have been measured using both relative and absolute methods. They were found to be (1.2±0.5)×10-10 and (2.9±1.1)×10-11 cm3 molecule−1 s−1 for α- and γ-terpinene respectively. Mechanistic studies have also been conducted in order to identify and quantify the main reaction products. Total organic nitrate and SOA yields have been determined. While organic nitrate formation yields appear to be similar, SOA yields exhibit large differences with γ-terpinene being a much more efficient precursor of aerosols. In order to provide explanations for this difference, chemical analysis of the gas-phase products was performed at the molecular scale. Detected products allowed for proposing chemical mechanisms and providing explanations through peroxy and alkoxy reaction pathways.

An incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS) technique has been developed for the in situ monitoring of NO3 radicals at the parts per trillion level in the CSA simulation chamber (at LISA). The technique couples an incoherent broadband light source centered at 662 nm with a high-finesse optical cavity made of two highly reflecting mirrors. The optical cavity which has an effective length of 82 cm allows for up to 3 km of effective absorption and a high sensitivity for NO3 detection (up to 6 ppt for an integration time of 10 s). This technique also allows for NO2 monitoring (up to 9 ppb for an integration time of 10 s). Here, we present the experimental setup as well as tests for its characterization and validation. The validation tests include an intercomparison with another independent technique (Fourier-transform infrared, FTIR) and the absolute rate determination for the reaction trans-2-butene + NO3, which is already well documented in the literature. The value of (4.13 ± 0.45) × 10−13 cm3 molecule−1 s−1 has been found, which is in good agreement with previous determinations. From these experiments, optimal operation conditions are proposed. The technique is now fully operational and can be used to determine rate constants for fast reactions involving complex volatile organic compounds (VOCs; with rate constants up to 10−10 cm3 molecule−1 s−1).

We present a digital, freely available, searchable, and evaluated compilation of rate coefficients for the gas-phase reactions of organic compounds with OH, Cl, and NO3 radicals and with O3. Although other compilations of many of these data exist, many are out of date, most have limited scope, and all are difficult to search and to load completely into a digitized form. This compilation uses results of previous reviews, though many recommendations are updated to incorporate new or omitted data or address errors, and includes recommendations on many reactions that have not been reviewed previously. The database, which incorporates over 50 years of measurements, consists of a total of 2765 recommended bimolecular rate coefficients for the reactions of 1357 organic substances with OH, 709 with Cl, 310 with O3, and 389 with NO3, and is much larger than previous compilations. Many compound types are present in this database, including naturally occurring chemicals formed in or emitted to the atmosphere and anthropogenic compounds such as halocarbons and their degradation products. Recommendations are made for rate coefficients at 298 K and, where possible, the temperature dependences over the entire range of the available data. The primary motivation behind this project has been to provide a large and thoroughly evaluated training dataset for the development of structure–activity relationships (SARs), whose reliability depends fundamentally upon the availability of high-quality experimental data. However, there are other potential applications of this work, such as research related to atmospheric lifetimes and fates of organic compounds, or modelling gas-phase reactions of organics in various environments. This database is freely accessible at https://doi.org/10.25326/36 (McGillen et al., 2019).

Multifunctional organic nitrates, including carbonyl nitrates, are important species formed in NOx-rich atmospheres by the degradation of volatile organic compounds (VOCs). These compounds have been shown to play a key role in the transport of reactive nitrogen and, consequently, in the ozone budget; they are also known to be important components of the total organic aerosol. However, very little is known about their reactivity in both the gas and condensed phases. Following a previous study that we published on the gas-phase reactivity of α-nitrooxy ketones, the photolysis and reaction with OH radicals of 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone (which are a β-nitrooxy ketone and γ-nitrooxy ketone, respectively) were investigated for the first time in simulation chambers. The photolysis frequencies were directly measured in the CESAM chamber, which is equipped with a very realistic irradiation system. The jnitrate/jNO2 ratios were found to be (5.9±0.9)×10-3 for 4-nitrooxy-2-butanone and (3.2±0.9)×10-3 for 5-nitrooxy-2-pentanone under our experimental conditions. From these results, it was estimated that ambient photolysis frequencies calculated for typical tropospheric irradiation conditions corresponding to the 1 July at noon at 40∘ N (overhead ozone column of 300 and albedo of 0.1) are (6.1±0.9)×10-5 s−1 and (3.3±0.9)×10-5 s−1 for 4-nitrooxy-2-butanone and 5-nitrooxy-2-pentanone, respectively. These results demonstrate that photolysis is a very efficient sink for these compounds with atmospheric lifetimes of few hours. They also suggest that, similarly to α-nitrooxy ketones, β-nitrooxy ketones have enhanced UV absorption cross sections and quantum yields equal to or close to unity and that γ-nitrooxy ketones have a lower enhancement of cross sections, which can easily be explained by the larger distance between the two chromophore groups. Thanks to a product study, the branching ratio between the two possible photodissociation pathways is also proposed. Rate constants for the reaction with OH radicals were found to be (2.9±1.0)×10-12 and (3.3±0.9)×10-12 cm3 molecule−1 s−1, respectively. These experimental data are in good agreement with rate constants estimated by the structure–activity relationship (SAR) of Kwok and Atkinson (1995) when using the parametrization proposed by Suarez-Bertoa et al. (2012) for carbonyl nitrates. Comparison with photolysis rates suggests that the OH-initiated oxidation of carbonyl nitrates is a less efficient sink than photodissociation but is not negligible in polluted areas.

A commercial PTR-TOF-MS has been optimized in order to allow the measurement of individual organic nitrates in the atmosphere. This has been accomplished by shifting the distribution between different ionizing analytes, H3O+∕ H3O+(H2O)n or NO+∕ NO2+. The proposed approach has been proven to be appropriate for the online detection of individual alkyl nitrates and functionalized nitrates. It has been shown that hydroxyl and ketonitrates have a high affinity towards NO+, leading to the formation of an adduct that allows the easy identification of the organic nitrate (R) from the R–NO+ ion signal. The recorded sensitivities for both ionization modes correspond to detection limits of tens of ppt min−1 in the case of hydroxy- and ketonitrates. Alkyl nitrates exhibit a moderate affinity towards NO+ ionization leading to detection units of few hundreds of ppt and the highest sensitivity in H3O+ mode was obtained for the water adducts signals. However, this method exhibits much lower capabilities for the detection of peroxyacetyl nitrates with detection limits in the ppb range.

Oxidation of biogenic volatile organic compounds (BVOC) by the nitrate radical (NO3) represents one of the important interactions between anthropogenic emissions related to combustion and natural emissions from the biosphere. This interaction has been recognized for more than 3 decades, during which time a large body of research has emerged from laboratory, field, and modeling studies. NO3-BVOC reactions influence air quality, climate and visibility through regional and global budgets for reactive nitrogen (particularly organic nitrates), ozone, and organic aerosol. Despite its long history of research and the significance of this topic in atmospheric chemistry, a number of important uncertainties remain. These include an incomplete understanding of the rates, mechanisms, and organic aerosol yields for NO3-BVOC reactions, lack of constraints on the role of heterogeneous oxidative processes associated with the NO3 radical, the difficulty of characterizing the spatial distributions of BVOC and NO3 within the poorly mixed nocturnal atmosphere, and the challenge of constructing appropriate boundary layer schemes and non-photochemical mechanisms for use in state-of-the-art chemical transport and chemistry–climate models. This review is the result of a workshop of the same title held at the Georgia Institute of Technology in June 2015. The first half of the review summarizes the current literature on NO3-BVOC chemistry, with a particular focus on recent advances in instrumentation and models, and in organic nitrate and secondary organic aerosol (SOA) formation chemistry. Building on this current understanding, the second half of the review outlines impacts of NO3-BVOC chemistry on air quality and climate, and suggests critical research needs to better constrain this interaction to improve the predictive capabilities of atmospheric models.

Secondary organic aerosol (SOA) were generated from the ozonolysis of α-pinene in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) simulation chamber. The SOA formation and aging were studied by following their optical, hygroscopic and chemical properties. The optical properties were investigated by determining the particle complex refractive index (CRI). The hygroscopicity was quantified by measuring the effect of relative humidity (RH) on the particle size (size growth factor, GF) and on the scattering coefficient (scattering growth factor, f(RH)). The oxygen to carbon atomic ratios (O : C) of the particle surface and bulk were used as a sensitive parameter to correlate the changes in hygroscopic and optical properties of the SOA composition during their formation and aging in CESAM. The real CRI at 525 nm wavelength decreased from 1.43–1.60 (±0.02) to 1.32–1.38 (±0.02) during the SOA formation. The decrease in the real CRI correlated to the O : C decrease from 0.68 (±0.20) to 0.55 (±0.16). In contrast, the GF remained roughly constant over the reaction time, with values of 1.02–1.07 (±0.02) at 90% (±4.2%) RH. Simultaneous measurements of O : C of the particle surface revealed that the SOA was not composed of a homogeneous mixture, but contained less oxidised species at the surface which may limit water absorption. In addition, an apparent change in both mobility diameter and scattering coefficient with increasing RH from 0 to 30% was observed for SOA after 14 h of reaction. We postulate that this change could be due to a change in the viscosity of the SOA from a predominantly glassy state to a predominantly liquid state.

A series of experiments was conducted in the CESAM (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) simulation chamber to investigate the evolution of the physical and chemical properties of secondary organic aerosols (SOAs) during different forcings. The present experiments represent a first attempt to comprehensively investigate the influence of oxidative processing, photochemistry, and diurnal temperature cycling upon SOA properties. SOAs generated from the ozonolysis of α-pinene were exposed under dry conditions (< 1% relative humidity) to (1) elevated ozone concentrations, (2) light (under controlled temperature conditions) or (3) light and heat (6 °C light-induced temperature increase), and the resultant changes in SOA optical properties (i.e. absorption and scattering), hygroscopicity and chemical composition were measured using a suite of instrumentation interfaced to the CESAM chamber. The complex refractive index (CRI) was derived from integrated nephelometer measurements of 525 nm wavelength, using Mie scattering calculations and measured number size distributions. The particle size growth factor (GF) was measured with a hygroscopic tandem differential mobility analyzer (H-TDMA). An aerosol mass spectrometer (AMS) was used for the determination of the f44 / f43 and O : C ratio of the particles bulk. No change in SOA size or chemical composition was observed during O3 and light exposure at constant temperature; in addition, GF and CRI of the SOA remained constant with forcing. On the contrary, illumination of SOAs in the absence of temperature control led to an increase in the real part of the CRI from 1.35 (±0.03) to 1.49 (±0.03), an increase of the GF from 1.04 (±0.02) to 1.14 (±0.02) and an increase of the f44 / f43 ratio from 1.73 (±0.03) to 2.23 (±0.03). The simulation of the experiments using the master chemical mechanism (MCM) and the Generator for Explicit Chemistry and Kinetics of Organics in the Atmosphere (GECKO-A) shows that these changes resulted from the evaporation of semi-volatile and less oxidized SOA species induced by the relatively minor increases in temperature (~ 6 °C). These surprising results suggest that α-pinene–O3 SOA properties may be governed more by local temperature fluctuations than by oxidative processing and photochemistry.

A new methodology for the determination of the changes due to hygroscopic growth with relative humidity of the number size distribution and optical properties of polydispersed aerosols is described. This method uses the simulation chamber CESAM where the hygroscopic properties of polydispersed aerosol particles can be investigated in situ by exposing them to RH ranging from 0 to 100% for approximately 1 h. In situ humidification is used to provide simultaneous information on the RH-dependence of the particle size and the corresponding scattering coefficient (σscat), and that for the entire size distribution. Optical closure studies, based on integrated nephelometer and aethalometer measurements, Mie scattering calculations and measured particle size distributions, can therefore be performed to yield derived parameters such as the complex refractive index (CRI) at λ = 525 nm. The CRI can also be retrieved in the visible spectrum by combining differential mobility analyzer (DMA) and white light aerosol spectrometer (Palas Welas®) measurements. We have applied this methodology to ammonium sulfate particles, which have well known optical and hygroscopic properties. The CRI obtained from the two methods (1.54–1.57) compared favourably to each other and are also in reasonable agreement with the literature values. The particle's growth was compared to values obtained for one selected size of particles (150 nm) with a H-TDMA and the effect of the residence time for particles humidification was investigated. When the humidification was performed in the chamber for a few minutes, a continuous increase of the ammonium sulfate particle's size and σscat was observed from RH values as low as 30% RH. Comparison of the measured and modelled values based on Köhler and Mie theories shows that layers of water are adsorbed on ammonium sulfate particles below the deliquescence point. In contradiction, the particle's growth reported with H-TDMAs showed a prompt deliquescence of ammonium sulfate particles with no continuous growth in size at low RH. These findings highlight the need to allow sufficient time for particle-water vapour equilibrium in investigating the aerosols hygroscopic properties. H-TDMA instruments induce limited residence time for humidification and seem to be insufficient for water adsorption on ammonium sulfate particles.

A new simulation chamber has been built at the Interuniversitary Laboratory of Atmospheric Systems (LISA). The CESAM chamber (French acronym for Experimental Multiphasic Atmospheric Simulation Chamber) is designed to allow research in multiphase atmospheric (photo-) chemistry which involves both gas phase and condensed phase processes including aerosol and cloud chemistry. CESAM has the potential to carry out variable temperature and pressure experiments under a very realistic artificial solar irradiation. It consists of a 4.2 m3 stainless steel vessel equipped with three high pressure xenon arc lamps which provides a controlled and steady environment. Initial characterization results, all carried out at 290–297 K under dry conditions, concerning lighting homogeneity, mixing efficiency, ozone lifetime, radical sources, NOy wall reactivity, particle loss rates, background PM, aerosol formation and cloud generation are given. Photolysis frequencies of NO2 and O3 related to chamber radiation system were found equal to (4.2 × 10−3 s−1) for JNO2 and (1.4 × 10−5 s−1) for JO1D which is comparable to the solar radiation in the boundary layer. An auxiliary mechanism describing NOy wall reactions has been developed. Its inclusion in the Master Chemical Mechanism allowed us to adequately model the results of experiments on the photo-oxidation of propene-NOx-Air mixtures. Aerosol yields for the α-pinene + O3 system chosen as a reference were determined and found in good agreement with previous studies. Particle lifetime in the chamber ranges from 10 h to 4 days depending on particle size distribution which indicates that the chamber can provide high quality data on aerosol aging processes and their effects. Being evacuable, it is possible to generate in this new chamber clouds by fast expansion or saturation with or without the presence of pre-existing particles, which will provide a multiphase environment for aerosol-droplet interaction.

The photooxidation of methacrolein was studied in the aqueous phase under simulated cloud droplet conditions. The obtained rate constant of OH-oxidation of methacrolein at 6°C in unbuffered solutions was 5.8(±0.9)×109 M−1 s−1. The measured rate coefficient is consistent with OH-addition on the C=C bond. This was confirmed by the mechanism established on the study of the reaction products (at 25°C in unbuffered solutions) where methylglyoxal, formaldehyde, hydroxyacetone and acetic acid/acetate were the main reaction products. An upper limit for the total carbon yield was estimated to range from 53 to 85%, indicating that some reaction products remain unidentified. A possible source of this mismatch is the formation of higher molecular weight compounds as primary reaction products which are presented in El Haddad et al. (2009) and Michaud et al. (2009).

Knowing the ozone absorption cross sections in the ultraviolet and infrared spectral range, with an accuracy of better than 1%, is of the utmost importance for atmospheric remote-sensing applications. For this reason, various ozone intensity intercomparisons and measurements have been published these last years. However, the corresponding results proved not to be consistent and thus have raised a controversial discussion in the community of atmospheric remote-sensing. This study, where great care has been taken to avoid any possible error, reports a new laboratory intercomparison of the ozone absorption coefficients in the mid-infrared (10 μm) and ultraviolet (300−350 nm) spectral regions. It gives a new piece of information to the puzzling problem concerning the ozone IR and UV cross sections and confirms that the IR and UV cross sections recommended in the literature are in disagreement of about 4%. [ABSTRACT FROM AUTHOR]

The kinetics and mechanisms of the gas-phase reactions of NO 3 radical with two branched unsaturated aldehydes, 2-methyl-2-butenal (also called 2-methyl-crotonaldehyde) and 3-methyl-2-butenal (or 3-methyl-crotonaldehyde), have been investigated by experimental and theoretical approaches. Kinetic data were also provided, for comparison, for 2-butenal (or crotonaldehyde). Experiments were performed in a simulation chamber at 295 ± 3 K and atmospheric pressure. Rate constants were determined using both absolute and relative rate methods. Experimental results were found to be in good agreement leading to the following rate constants (in cm 3 molecule -1 s -1 ): k (2-butenal + NO 3 ) = (4.6 ± 1.3) × 10 -15 ; k (2-methyl-2-butenal + NO 3 ) = (14.0 ± 2.8) × 10 -15 ; and k (3-methyl-2-butenal + NO 3 ) = (19.1 ± 4.1) × 10 -15 . Theoretical calculations were also performed using the DFT-BH&HLYP/6-311++G(d,p) method and lead to rate constants in agreement with experiments and allow us to explore mechanisms for abstraction and addition pathways. Impact on atmospheric chemistry is discussed.

Particulate matter from biomass burning emissions affects air quality, ecosystems and climate; however, quantifying these effects requires that the connection between primary emissions and secondary aerosol production is firmly established. We performed atmospheric simulation chamber experiments on the chemical oxidation of residential biomass burning emissions under dark conditions. Biomass burning organic aerosol was found to age under dark conditions, with its oxygen-to-carbon ratio increasing by 7-34% and producing 1-38 μg m -3 of secondary organic aerosol (5-80% increase over the fresh organic aerosol) after 30 min of exposure to NO 3 radicals in the chamber (corresponding to 1-3 h of exposure to typical nighttime NO 3 radical concentrations in an urban environment). The average mass concentration of SOA formed under dark-oxidation conditions was comparable to the mass concentration formed after 3 h (equivalent to 7-10 h of ambient exposure) under ultraviolet lights (6 μg m -3 or a 47% increase over the emitted organic aerosol concentration). The dark-aging experiments showed a substantial increase in secondary nitrate aerosol (0.12-3.8 μg m -3 ), 46-100% of which is in the form of organic nitrates. The biomass burning aerosol pH remained practically constant at 2.8 throughout the experiment. This value promotes inorganic nitrate partitioning to the particulate phase, potentially contributing to the buildup of nitrate aerosol in the boundary layer and enhancing long-range transport. These results suggest that oxidation through reactions with the NO 3 radical is an additional secondary aerosol formation pathway in biomass burning emission plumes that should be accounted for in atmospheric chemical-transport models., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

The influence of the precursor chemical structure on secondary organic aerosol (SOA) formation was investigated through the study of the ozonolysis of two anthropogenic aromatic alkenes: 2-methylstyrene and indene. Experiments were carried out in three different simulation chambers: ICARE 7300L FEP Teflon chamber (ICARE, Orléans, France), EUPHORE FEP Teflon chamber (CEAM, Valencia, Spain), and CESAM evacuable stainless steel chamber (LISA, Créteil, France). For both precursors, SOA yield and growth were studied on a large range of initial concentrations (from ∼60 ppbv to 1.9 ppmv) and the chemical composition of both gaseous and particulate phases was investigated at a molecular level. Gas phase was described using FTIR spectroscopy and online gas chromatography coupled to mass spectrometry, and particulate chemical composition was analyzed (i) online by thermo-desorption coupled to chemical ionization mass spectrometry and (ii) offline by supercritical fluid extraction coupled to gas chromatography and mass spectrometry. The results obtained from a large set of experiments performed in three different chambers and using several complementary analytical techniques were in very good agreement. SOA yield was up to 10 times higher for indene ozonolysis than for 2-methylstyrene ozonolysis at the same reaction advancement. For 2-methylstyrene ozonolysis, formaldehyde and o-tolualdehyde were the two main gaseous phase products while o-toluic acid was the most abundant among six products detected within the particulate phase. For indene ozonolysis, traces of formic and phthalic acids as well as 11 species were detected in the gaseous phase and 11 other products were quantified in the particulate phase, where phthaldialdehyde was the main product. On the basis of the identified products, reaction mechanisms were proposed that highlight specific pathways due to the precursor chemical structure. These mechanisms were finally compared and discussed regarding SOA formation. In the case of 2-methylstyrene ozonolysis, ozone adds mainly on the external and monosubstituted double bond, yielding only one C8- and monofunctionalized Criegee intermediate and hence more volatile products as well as lower SOA mass than indene ozonolysis in similar experimental conditions. In the case of indene, ozone adds mainly on the five-carbon-ring and disubstituted C═C double bond, leading to the formation of two C9- and bifunctionalized Criegee intermediates, which then evolve via different pathways including the hydroperoxide channel and form highly condensable first-generation products.

Laboratory studies of atmospheric chemistry characterize the nature of atmospherically relevant processes down to the molecular level, providing fundamental information used to assess how human activities drive environmental phenomena such as climate change, urban air pollution, ecosystem health, indoor air quality, and stratospheric ozone depletion. Laboratory studies have a central role in addressing the incomplete fundamental knowledge of atmospheric chemistry. This article highlights the evolving science needs for this community and emphasizes how our knowledge is far from complete, hindering our ability to predict the future state of our atmosphere and to respond to emerging global environmental change issues. Laboratory studies provide rich opportunities to expand our understanding of the atmosphere via collaborative research with the modeling and field measurement communities, and with neighboring disciplines.

The temperature and concentration dependence of secondary organic aerosol (SOA) yields has been investigated for the first time for the photooxidation of n-dodecane (C 12 H 26 ) in the presence of NO x in the CESAM chamber (French acronym for "Chamber for Atmospheric Multiphase Experimental Simulation"). Experiments were performed with and without seed aerosol between 283 and 304.5 K. In order to quantify the SOA yields, a new parametrization is proposed to account for organic vapor loss to the chamber walls. Deposition processes were found to impact the aerosol yields by a factor from 1.3 to 1.8 between the lowest and the highest value. As with other photooxidation systems, experiments performed without seed and at low concentration of oxidant showed a lower SOA yield than other seeded experiments. Temperature did not significantly influence SOA formation in this study. This unforeseen behavior indicates that the SOA is dominated by sufficiently low volatility products for which a change in their partitioning due to temperature would not significantly affect the condensed quantities.

The density functional theory with the BH&HLYP functional has been used in this work to clarify discrepancies found in the literature about the effect of the increasing carbon chain on the reactivity of trans-2-alkenals from acrolein (C3) to trans-2-octenal (C8) with nitrate radical. In this work, it was found that (i) the alkyl chain length of the unsaturated aldehydes has little or no influence on the NO3 reaction rate coefficients (ii) the abstraction of the aldehydic hydrogen from the alkenal is always dominant (83% for trans-2-butanal to trans-2-octenal). The addition channel, which mainly concerns the β addition, has a small influence (17% of the total reaction for the whole series). These results are in good agreement with the experimental studies performed by Zhao et al. in 2011 and by Kerdouci et al. in 2012. All these findings will be useful to complete or improve structure-activity relationships developed to predict the reactivity of NO3 radicals with organic compounds.

A new sensitive technique for the quantification of formaldehyde (HCHO) and total aldehydes has been developed in order to monitor these compounds, which are known to be involved in air quality issues and to have health impacts. Our approach is based on a colorimetric method where aldehydes are initially stripped from the air into a scrubbing solution by means of a turning coil sampler tube and then derivatised with 3-methylbenzothiazolinone-2-hydrazone in acid media (pH = -0.5). Hence, colourless aldehydes are transformed into blue dyes that are detected by UV-visible spectroscopy at 630 nm. Liquid core waveguide LCW Teflon® AF-2400 tube was used as innovative optical cells providing a HCHO detection limit of 4 pptv for 100 cm optical path with a time resolution of 15 min. This instrument showed good correlation with commonly used techniques for aldehydes analysis such as DNPH derivatisation chromatographic techniques with off-line and on-line samplers, and DOAS techniques (with deviation below 6%) for both indoor and outdoor conditions. This instrument is associated with simplicity and low cost, which is a prerequisite for indoor monitoring.

Multifunctional organic nitrates are potential NO(x) reservoirs whose atmospheric chemistry is somewhat little known. They could play an important role in the spatial distribution of reactive nitrogen species and consequently in ozone formation and distribution in remote areas. In this work, the rate constants for the reaction with OH radical and the photolysis frequencies of α-nitrooxyacetone, 3-nitrooxy-2-butanone, and 3-methyl-3-nitrooxy-2-butanone have been determined at room temperature at 1000 mbar total pressure of synthetic air. The rate constants for the OH oxidation were measured using the relative rate technique, with methanol as reference compound. The following rate constants were obtained for the reaction with OH: k(OH) = (6.7 ± 2.5) × 10(-13) cm(3) molecule(-1) s(-1) for α-nitrooxyacetone, (10.6 ± 4.1) × 10(-13) cm(3) molecule(-1) s(-1) for 3-nitrooxy-2-butanone, and (2.6 ± 0.9) × 10(-13) cm(3) molecule(-1) s(-1) for 3-methyl-3-nitrooxy-2-butanone. The corresponding photolysis frequencies extrapolated to typical atmospheric conditions for July first at noon at 40° latitude North were (4.8 ± 0.3) × 10(-5) s(-1), (5.7 ± 0.3) × 10(-5) s(-1), and (7.4 ± 0.2) × 10(-5) s(-1), respectively. The data show that photolysis is a major atmospheric sink for these organic nitrates.

Rate constants for the gas-phase reactions of the NO(3) radical with a series of unsaturated aldehydes, trans-2-hexenal, trans-2-heptenal, and trans-2-octenal, have been measured using absolute rate method at 294 ± 3 K and atmospheric pressure. This work was performed to clarify discrepancies found in the literature and thus led to a clearer view of the effect of the increasing carbon chain length on the reactivity of trans-2-alkenals. The rate constants were determined to be (4.7 ± 1.5) × 10(-15), (5.3 ± 1.6) × 10(-15), and (5.6 ± 2.3) × 10(-15) cm(3) molecule(-1) s(-1) for trans-2-hexenal, trans-2-heptenal, and trans-2-octenal, respectively. These results clearly indicate that the carbon chain lengthening of the trans-2-alkenals does not significantly affect the rate constant. In addition, the mechanism for the reaction of NO(3) with these unsaturated aldehydes was also investigated. Unsaturated peroxynitrate-type compounds that are exclusively formed through the abstraction channel were observed as the main products.

A new structure-activity relationship (SAR), based on parametrization of the molecular structure according to the group-additivity method, is presented. On the basis of existing experimental data for the degradability of approximately 150 organic compounds by the NO(3) radical, this new SAR is developed to estimate the rate constants for reactions with NO(3) radical. At night, nitrate radicals are the most important oxidant of volatile organic compounds. The rate constants for their reactions are therefore essential to the understanding of VOC degradation and atmospheric modelling. The database used for the SAR development includes most classes of compounds such as alkanes, alkenes (acyclic and cyclic), dienes, terpenes and saturated and unsaturated oxygenated compounds (including alcohols, ketones, ethers and esters). The proposed SAR shows good efficiency, as 91% of the rate constants are reproduced within a factor of two. The overall agreement between measured and predicted rate constants is very good for most of the unsaturated and saturated compounds, although for saturated alcohols it is less reliable.

Vinyl acetate is widely used in industry. It has been classified as a high-production volume (HPV) chemical in the United States. To evaluate its impact on the environment and air quality, its atmospheric reactivity toward the three main tropospheric oxidants (OH, NO(3), and O(3)) has been investigated. Kinetic and mechanistic experiments have been conducted at room temperature and atmospheric pressure using an indoor Pyrex simulation chamber coupled to Fourier transform infrared (FTIR) and UV-visible spectrometers. Rate constants for the reactions of vinyl acetate with OH, NO(3), and O(3) were equal to (2.3 +/- 0.3) x 10(-11), (7.3 +/- 1.8) x 10(-15), and (3.0 +/- 0.4) x 10(-18) cm(3) molecule(-1) s(-1), respectively. From these data, tropospheric lifetimes of vinyl acetate have been estimated as follows: tau(OH) = 6 h, tau(NO(3)) = 6 days, and tau(O(3)) = 5 days. This demonstrates that reaction with OH radicals is the main tropospheric loss process of this compound. From the mechanistic experiments, main oxidation products have been identified and quantified and oxidation schemes have been proposed for each studied reaction.

UV (240-370 nm) and IR (3200-1500 cm(-1)) absorption cross-sections of HCHO, HCDO, and DCDO in a bath gas of N(2) at atmospheric pressure and 296 K are reported from simultaneous measurements in the two spectral regions. Cross-sections were placed on an absolute scale through quantitative conversion of formaldehyde to CO and HCOOH by titration with Br atoms, also monitored by FTIR. The integrated UV absorption cross-sections of HCHO, HCDO, and DCDO are equal to within the experimental uncertainty.

NO(3) oxidation of methyl, ethyl, propyl, and butyl vinyl ethers has been studied under tropospheric conditions (atmospheric pressure and T = 293 +/- 3 K) in the LISA indoor simulation chamber. NO(3) was produced inside the reactor by thermal decomposition of N(2)O(5) previously added to the air-VOC mixture, and concentrations were monitored using FTIR spectrometry. All the kinetic experiments were carried out by relative rate technique using isoprene as reference compound, leading to the rate constants k(1) = (7.2 +/- 1.5) x 10(-13), k(2) = (13.1 +/- 2.7) x 10(-13), k(3) = (13.3 +/- 3.0) x 10(-13), and k(4) = (17.0 +/- 3.7) x 10(-13) cm(3) molecule(-1) s(-1) for methyl, ethyl, propyl, and butyl vinyl ethers, respectively. Main oxidation products have been identified like being formaldehyde and respectively methyl, ethyl, propyl, and butyl formates. Production yields of oxidation products were close to 50%. Oxygenated nitrates and peroxynitrates were also detected.

Acetates are emitted into the atmosphere by several anthropic and natural sources. To better evaluate the environmental impact of these compounds, OH-induced oxidation kinetic and mechanism of allyl acetate (CH3C(O)OCH2-CH=CH2) have been investigated at room temperature and atmospheric pressure using three environmental chambers: an indoor Teflon-film bag (LISA, Créteil), an indoor Pyrex photoreactor (LISA, Créteil), and the outdoor Smog chamber EUPHORE (Valencia). Rate constant of the reaction of allyl acetate with OH radicals was determined by relative rate technique in the indoor Teflon-film bag. It is (30.6 +/- 3.1) x 10(-12) cm3 molecule-1 s-1. Mechanistic experiments were performed in the indoor photoreactor and in the outdoor Smog chamber EUPHORE. The main oxidation products observed by FTIR in both chambers were acetoxyacetaldehyde and formaldehyde. From these data, a mechanism was developed to describe the OH-induced oxidation of this acetate in the presence of NOx. Finally, atmospheric impact of allyl acetate emissions was evaluated using kinetic and mechanistic results.